Method of bringing a fluidized catalytic cracker-regenerator system on stream



1961 J c DYGERT 3,012,962

METHOD OF BRINOINC A FLUIDIZED CATALYTIC CRACKER-REGENERATOR SYSTEM ONSTREAM Filed Aug. 23, 1954 2 Sheets-Sheet 1 WATER REGENERATOR TOFRACTIONATOR CATALYST STORAGE TO STACK FEED ou AIR HEATER AUXILIARYCOMBUSTION TOWER L J HEAT 42 3s 41 BOILER COMPRESSORS F I I INVENTORJUSTIN c. DYGERT METHOD F BRINGING A FLUIDEZED CATA- LYTICCRACKER-REGENERATOR SYSTEM 0N STREAM Justin C. Dygert, Walnut Creek,Qalifi, assignor to Shell Oil oxnpany, a corporation of Delaware FiledAug. 23, 1954, Ser. No. 451,519 3 Claims. (Cl. 208-154) Thisspecification which is directed to those skilled in the arts ofcatalytic cracking and gas turbine operation, relates to an inventionrelating to the catalytic cracking of hydrocarbon oils in aself-sustained operation involving the use of a gas turbine, and moreparticularly to better ways and means for bringing catalytic crackingunits on stream and for effecting the regeneration of the catalysttherein.

It will be appreciated by those skilled in the art of catalytic crackingthat, in the catalytic cracking of hydrocarbon oils using the fluidizedcatalyst technique, the used and partially spent powdered catalyst iscontinuously cycled through a separate regeneration zone wherein thecarbonaceous deposits are burned with air. It is the necessary and usualpractice to control the temperature during the burning between about1000 F. and 1200 F. such that the catalyst is not damaged byoverheating. For example, in the fluidized catalyst catalytic crackingprocess the temperature in the regeneration zone is normally maintainedat about 1100 F. At temperatures of this order the rate of burning isquite low; consequently, the residence time of the catalyst in theregeneration zone is fairly long and this requires the use of very largevessels containing many tons of catalyst. For example, many fluidizedcatalyst catalytic cracking plants have regeneration vessels about 40feet in diameter and such vessels may operate with around 600 tons ofcatalyst.

In normal operation, the air required for the combustion in theregeneration zone is compressed, passed through the regeneration zone,and is then discharged as flue gas to the stack. In processes using thefluidized catalyst technique the flue gas is generally passed throughone or more cyclone separators to remove suspended catalyst particlesbefore being discharged and in some of the earlier plants the cycloneseparators were followed by a Cottrell precipitator to improve therecovery of suspended catalyst. It is generally recognized that the costof compressing the enormous quantities of air required (e.g., to burntons of coke per hour) represents a major cost item in the catalyticcracking operation.

It will also be appreciated that it has been previously suggested topass the regenerator fine gas through a gas turbine prior to dischargingit to the stack and to utilize the work produced by the turbine tocompress air for the regeneration. It is recognized that this would be adesirable end if it could be made to work in practice. In the normaloperation the air is compressed by steam power. The boiler requirementsat the usual figure of about nine dollars per pound per hour of steamrepresent a large capital investment. The capital cost of gas turbinefacilities is of about the same order of magnitude; therefore, little ifanything can be gained by the application of the gas turbine unless thesystem with the gas turbine is self-sustaining, i.e., the useful workproduced by the turbine is capable of pumping all the required air.

It is known that a self-sustained system can be designed provided thatthe system is operated under a suitable high pressure and such unitsinvolving a plurality of small fixed beds of catalyst granules have beenbuilt. However, as a practical matter, very important engineer- ;ingproblems are encountered in attempts to construct elevated vessels ofthe large size required in fluidized 3,912,962 Patented Dec. 12, 1961catalyst catalytic cracking plants when such vessels are intended tooperate at temperatures of about 1100 F. under any appreciable pressure.Therefore, in spite of the advantages apparent in a self-sustained fluidcatalyst operation, no such operation is in use.

It has now been found that the disadvantages heretofore encountered maybe avoided, thereby providing means for a self-sustained operation, inunits operating at a low pressure where self-sustained operation wouldotherwise be impossible. The process of the invention is applicable influidized catalyst catalytic cracking systems when the regenerator fluegas is in the usual temperature range and the back pressure available.for the gas turbine is as low as between 6 and 10 p.s.i.g. In normaloperation, the corresponding exit pressures at the .air blower outletare between about 20 and 26 p.s.i.g., e.g., around 23 p.s.i.g. for an8.8 p.s.i.g. gas turbine inlet pressure.

While the process and arrangement of the apparatus of the invention areparticularly suitable for such low pressure catalytic cracking systems,they may also be applied in catalytic cracking systems in which theregenerator exit flue gas pressure is higher, e.g., in the range of 10to 70 p.s.i.g. i

In addition to providing a self-sustained operation, the gas turbinearrangement must be capable of bringing the plant on-stream withoutrequiring a large auxiliary system for start-up. If there is adequatecompressor capacity and a large auxiliary power source, no particulardifliculty is encountered in bringing the plant onstream. However, witha plant which is self-sustained during normal operation with little orno excess power, the normal method of bringing such a plant on-streamcannot be used without such a large auxiliary power supply and even thenthe bringing of such a plant onstream is a long and costly procedure. Itis obviously most desirable to be able to bring the plant on-stream inthe minimum time.

In the process and apparatus of the invention, the apparatus andarrangement of flows are arranged in a novel manner to allow thecatalytic cracking system to be brought on-stream in a minimum of timewith little or no extra facilities other than the usual starting motorused to start the gas turbine-compressor system.' In fact, the systemand method of the invention allow such a plant to be'brought on-streamin a shorter time than the conventional start-up procedure using theconven tional steam-driven air compressor.

The invention will be described with reference to a particularembodiment. In this description, reference will be had to theaccompanying drawing.

FIG. I shows a catalytic cracking system arranged to operate in themanner of the invention. The main pieces of equipment are indicateddiagrammatically.

FIG. ll illustrates a novel auxiliary combustion tower which may beadvantageously substituted for theauxiliary I combustion towerillustrated in FIG. I. ,Like parts are designated by the same referencenumbers.

The catalytic cracking plant illustrated is designed for low pressureoperation. Thus, the pressure of the regenerator flue gas at the gasturbine intake is only about 8.8 p.s.i.g. Referring to FIG. I, thesystem comprises a fluid catalyst regenerator l, a fluid catalystreactor 2, a catalyst storage vessel 3, an air heater 4, an auxiliarycombustion tower 5, a gas turbine 6, a low pressure air compressor 7,high pressure air compressors 8 and and a starting motor 10. While thecompressors 8 and 9 are shown diagrammatically as turbo compressorsoperating in parallel and driven from the common shaft through gears 37,it will be appreciated that a single second stage compressor may, besubstituted if desired.

3 with a particular flow of the flue gas and air as will be described inmore detail with reference to the operation.

When the plant is shut down for regularly scheduled maintenance, or forother reasons, the catalyst is transferred from the reactor andregenerator to the storage vessel 3. At the start, therefore, theregenerator and reactor are cold, at substantially atmospheric pressure,and empty of catalyst. In starting up, valves 15, 16, 17 and 18, whichcontrol the flow of air from the compressors to the auxiliary combustiontower, are opened and valves 11, 19, 40 and 46 are closed. Then the gasturbine and compressors, which may be either of the axial or the radialtype, are started with the starting motor 10 and a suitable fuel, e.g.,natural gas, is supplied to the auxiliary combustion tower 5 by line2t). The mixture is ignited by an ignitor (not shown) and valves 15 and16 are adjusted to maintain a steady combustion. The turbine acceleratesas the temperature of the gases in line 21 increases until at atemperature, e.g., about SON-800 E, the system becomes self-sustaining.At this point, the starting motor may be disconnected. The startingmotor which may be driven by electricity, gas, gasoline, steam, or thelike, may be quite small. Thus, a prime mover having a rated horse powerof as little as 5% of the normal operating horse power of the gasturbine can be used. In this stage of the start-up all of the highpressure air and low pressure air from the compressors 7, 8, and 9 iscycled to the auxiliary combustion tower.

When the turbine-compressor system is self-sustaining, and with valves12, 13 and 14 closed, the temperature of the gases in line 21 isincreased by increasing the fuel supplied via lines 20, and valve 19 isopened slightly, thereby forcing a portion of the high pressure air vialine 22 to the air heater 4 and from there to the regenerator andreactor vessels. The pressure in the reactorregenerator system isthereby gradually increased. As this pressure increases, valve 17controlling the flow of high pressure air to the auxiliary combustiontower is gradually throttled. When the pressure in the regeneratorreaches substantially the pressure prevailing in the auxiliarycombustion tower 5, valve 11 is opened thereby establishing circulationof air through the reactor-regenerator system. Valve 17 is thencompletely closed. In this second stage of the start-up, part of thehigh pressure air from compressors 8 and 9 is passed to the closedreactor-regenerator system whereas the remainder of the high pressureair is passed along with the low pressure air to the auxiliarycombustion tower. If desired, valve 41 may be opened during part of thisperiod and valve 42 may be closed. Thus, the hot exhaust from theturbine may be passed by lines 22, 24 and 25 to preheat the regeneratoruntil the back pressure makes it advisable to close valve 41 and openvalve 42 thereby passing the hot exhaust gases through the waste heatboiler to the stack. Also some further economy can be gained during thisperiod by burning fuel in the air heater 4 and venting the regeneratorexit gases directly to the stack through valve 13.

At the end of this second stage of the startup, the pressure in thecatalytic cracking system will be seen to be intermediate the normaloperating pressures. Thus, the pressure in the auxiliary combustiontower is above the normal operating pressure, whereas the pressure inthe riser 25 is much below the normal operating pressure.

For example, in the particular case in question the nonnal operatingpressure in the auxiliary combustion tower is about 8.8 p.s.i.g. and thenormal operating pressure in the heater '4 is about 23 p.s.i.g. At thisstage of the startup in the example, the respective pressures are about10.0 and 13.1 p.s.i.g. These pressure differences afford increased powercapacity as well as a large increase in compression capacity which canbe used to pump large masses of air through the system and thereby heatit to temperature in the shortest possible time. It is here not aquestion of preheating the gas but of heating the large masses ofequipment.

At this point either of two procedures may be followed. According to thefirst, valve 11 may be throttled to maintain the pressure in theauxiliary combustion tower below the discharge pressure of the lowpressure compressor 7 whereby the pressure in the regenerator andreactor is gradually increased to the normal operating pressure asmeasured at or near the heater 4. According to the alternativeprocedure, valve 18 is closed and valve 40 is opened (valve 41 beingclosed) whereby all of the air is passed through the catalytic crackingsystem without any appreciable increase in pressure therein.

In either case, a suitable fuel, e.g., natural gas, is introduced intoheater 4 via line 23 and ignited as soon as the airflow through theheater is sufficient to support combustion. The amount of fuel isadjusted such that the outlet temperature of the heated air is of theorder of 1100- 1350 F. Thus, heated air is circulated through thecracking system, the auxiliary combustion tower, and the gas turbine toheat the catalytic cracking system while additional heat is supplied bythe fuel introduced by line 20.

The valves in the standpipe lines 32 and 33 may be closed and steam maybe injected by line 34 to heat and purge the reactor. The vapors may bevented through valve 14.

At any desired time after preheating of the cracking system has beeninitiated, catalyst is withdrawn from the catalyst storage vessel 3 vialine 35 and valve 36 and passed to the regenerator via line 24 and riser25. In the first case mentioned above, where valve 11 is throttled, thisvalve is gradually opened as the catalyst is charged. Thus, in thiscase, the normal design pressure drop through the catalytic crackingsystem is maintained by throttling valve 11 and, as the normal pressuredrop is imposed by the introduction of catalyst, the pressure dropthrough valve 13 is decreased thereby providing a smooth transition. Inthe second case mentioned above, valve 40 is closed and valve 18 isopened as catalyst is added such that the pressure at valve 11 remainssubstantially constant whereas the pressure measured near the preheater4 gradually increases to overcome the increased pressure drop caused bythe catalyst introduced.

The heating of the regenerator and catalyst therein is continued withthe hot air until the temperature is sufficient to initiate combustionat which time a suitable fuel, e.g., torch oil, is introduced directlyto the catalyst bed by lines 27. From this point on the temperature inthe regenerator increases relatively rapidly to the design operatingtemperature, e.g., 1100 F. It will usually be necessary to cut back oraltogether stop the injection of fuel to the auxiliary combustion towerby line 20 in order to maintain the temperature of the gas in line 21 atthe desired turbine inlet temperature, e.g., about 1300 F. Also, as theregenerator approaches the normal operating temperature the injection offuel to the air heater 4 is discontinued. At any time during this stageof the startup, circulation of the catalyst through the regenerator andreactor may be started. This catalyst circulation is effected in anormal manner except that steam or natural gas is supplied by line 34instead of feed oil. When the desired temperature and circulation areachieved, oil feed may be injected via line 34 to bring the uniton-stream. The injection of torch oil via lines 27 may then be curtailedor discontinued.

Also when the temperature in the regenerator ap proaches the desiredoperating temperature, a fine spray of water or steam is introduced vialine 29 into the disengaging space in the top portion of the regeneratorabove the fluidized bed of catalyst. In fluidized catalyst regeneratorsof this type, it is found that part of the combustion tends to takeplace in the gas phase above the catalyst bed. It is essential in thepresent operation in a low pressure system such as described thatcombustion in this gas phase zone be curtailed. If combustion takesplace in this zone either continuously or at intervals the amount of airpassed to the auxiliary combustion tower must be reduced in order tomaintain the desired outlet temperature and steady operation of theauxiliary combustion tower becomes diflicult if not impossible. Theamount of water or steam injected by line 29 is adjusted to prevent thegas phase temperature in the region of the cyclone separators 30 fromexceeding about 1150 F. Thus, in normal operation the gases leaving thefluidized bed of catalyst 31 are cooled by the steam injected orproduced by vaporization of the water spray in the gas phase above thecatalyst bed. If water is injected to affect this cooling, it ispreferably condensate Water which is low in mineral matter.

When operating as described, the regenerator flue gas leaving theregenerator by line 26 normally contains a small amount of carbonmonoxide, e.g., about 6%. This small amount of carbon monoxide is burnedin the auxiliary combustion tower 5 and results in an increase in theflue gas temperature. The amount of air introduced through valve 16 isadjusted such that the auxiliary combustion tower exit gas does notexceed the safe maximum turbine temperature. if, for any reason, thecarbon monoxide content of the flue gas entering by line 26 is notsuflicient to support combustion, a small bleed of natural gas may beintroduced by line 20 to give a combustion supporting mixture. This is,however, not necessary when water injection by line 29 is properlyadjusted and a normal cracking catalyst is employed in the process.

As stated above, FIG. II illustrates a novel auxiliary combustion towerwhich may be advantageously substituted for the auxiliary combustiontower illustrated in FIG. I. This tower is provided in the fore-section(in this case the upper section) with an oxidation catalyst such, forexample, as the platinum coated ceramic oxidation catalyst sold underthe name Oxycat (See The Oil and Gas Journal, June 7, 1954, page 99).The fuel burners are located in the after-section (in this case thelower section). The piping is arranged such that the incoming air may bepassed either to the fore-section or to the aftersection, or may besplit between these sections by con trolling valves 15 and 16. Suitableintercomunicating ports are provided between the sections, e.g., theports 44 in the checker brick construction in the lower section, suchthat the gas entering the tower through valves 11 and 15 passes throughthe fore-section and then through the after-section, whereas gasintroduced through valve 16 passes only through the after-section.

When bringing the plant on-stream the air stream is split such that atleast a part of the air is passed through valve 15. This air passes downthrough the zone 43 packed with catalyst thereby cooling the catalystand preventing it from being overheated by heat from the lowercombustion zone. The remaining air required for the combustion of thefuel injected by lines 20, as well as that required to cool thecombustion gases to a safe tem perature for the gas turbine, isintroduced through valve 16. In order to facilitate stabilization of theflame in the after-sectionin the presence of the large excess of air,this air may be in part introduced through ports 45 in the chamber walldownstream of the flame.

It will be appreciated that, while the auxiliary combustion towerdescribed has the catalyst supported in the upper section and operateswith downflow of gas, the apparatus may be built for upflow of gas withthe catalyst in the lower section, or it may be constructed in ahorizontal position. The downflow arrangement illustrated affords theleast pressure drop between valve 11 and line 21. While it is importantto minimize this pressure drop in systems such as in this specificexample where the flue gas pressure is of the order of 2 to p.s.i.g., itis less important in applications where the flue gas pressure is higher.

In the foregoing, the bringing of the catalytic cracking unit on-streamhas been described. When the system is on-stream, it is self-sustainingand substantially in balance;

that is the power requirements for the process are satisfied withlittle, if any, power to spare. The system then operates as follows:Motor 10 is not operating. Valves 13, 14,

17, 36, 40, 41 and 46 are closed. Valves 11, 12, 18, 19, and 42 areopen. Valves -15 and/ or 16 are open. No fuel is supplied via line 23and little if any fuel is supplied by lines 20 or 27. Water is injectedby line 29 as needed. Part of the low pressure air from compressor 7 ispassed by valves 18 and 15 and/or 16 to the auxiliary combustion towerwhile the remainder is further compressed by compressors 8 and ,9 andpassed through valve 19, line 22, heater 4 (which is no longer in use asa heater), line 24, and riser 25 to the regenerator to supply the needsthereof. The flue gas, after passing through the cyclone separators 30,passes via line 26 and valve 11 to the auxiliary combustion towerwherein the residual carbon monoxide is burned in the presence of airintroduced through valve 15, and the resulting flue gas, after beingtempered with air introduced through valve 16, is passed via line 21 tothe turbine 6. The turbine exhaust gases which may be, for example, 200F. below the turbine inlet gases, are passed through valve 42 to thewaste heat broiler 39, which is optional and from there to the stack.The operation of the cracking reactor is not pertinent to the inventionand is deemed to be obvious to those skilled in the art; it is thereforenot discussed.

I claim as my invention:

1. In bringing a catalytic cracking plant on-stream, said catalyticcracking plant comprising a fluidized catalyst regenerator and aseparate fluidized catalyst reactor, the

combination of steps which comprises closing said regenerator againstescape of gas, compressing heated air into said regenerator until thepressure equals substantially the normal operating pressure at theregenerator air inlet whereby the pressure at the regenerator flue gasoutlet is substantially above the normal operating pressure,transferring powdered catalyst to said regenerator in suspension in saidair, and decreasing the back pressure on the regenerator flue gas outletas the catalyst is introduced thereby maintaining a substantiallyconstant pressure drop in the air passed through said regenerator.

2. In the catalytic cracking of a hydrocarbon oil with a finely dividedcracking catalyst wherein the used catalyst is continuously regeneratedin a separate regeneration zone at a temperature of the order of 1000 to1200 F. by burning carbonaceousdeposits therefrom with air with theproduction of a flue gas at a low pressure in the range of 6 to 10p.s.i.g. and containing carbon monoxide, the improvement which comprisescompressing air to an intermediate pressure which is at least equal tothe pressure of said flue gas, and with useful work as hereinafterdescribed, combining part of said compressed air with the regeneratedflue gas under conditions to combust the carbon monoxide therein, mixingwith the combustion gases an additional part of said compressed air inan amount to cool the combustion gases to about 1300" F., passing thecooled combustion gases to a gas expansion engine to generate usefulwork, utilizing part'of said useful work for aforesaid compression ofair to an intermediate pressure, utilizing the remainder of said usefulwork to compress further a third part of the air which is passed to theregeneration zone to supply'all of the air required for saidregeneration.

3. In the catalytic cracking of a hydrocarbon with a finely dividedcracking catalyst wherein the used catalyst is continuously regeneratedin a separate regeneration.

zone at a temperature of the order of 1000" to 1200 F. by burningcarbonaceous deposits therefrom with air with the production of a fluegas at a low pressure in the range of 6 to 10 p.s.i.g. and containingcarbon monoxide, the

improvement which comprises passing said flue gas at said pressure to anauxiliary combustion zone underconditions to burn the carbon monoxidetherein to heat the flue gas to a temperature of about 1300 F., passingthe heated gas to a gas expansion engine to generate useful work,utilizing part of said useful work to compress air to an vsure of saidflue gas, passing a portion of said air at intermediate pressure to theauxiliary combustion zone, utilizing the remainder of said useful workto compress further the remainder of the air from the intermediatepressure to a final pressure above the pressure of the flue gas andpassing the air at said final pressure to the regeneration zone as allof the air required for regeneration.

Vose Aug. 1, 1939 Ramseyer Nov. 27, 1945 8, 2,391,366 Tyson Dec. 18,1945 2,449,096 Wheeler Sept. 14, 1948 2,758,979 Guthrie Aug. 14, 1956 5FOREIGN PATENTS 973,589 France Feb. 12, 1951 OTHER REFERENCES Arden etal.: Disposal of Refinery Waste Gases, Oil

and Gas Journal, pages 99 to 101, page 109, June 7, 1954.

1. IN BRINGING A CATALYTIC CRACKING PLANT ON-STREAM, SAID CATALYTICCRACKING PLANT COMPRISING A FLUIDIZED CATALYST REGENERATOR AND ASEPARATE FLUIDIZED CATALYST REACTOR, THE COMBINATION OF STEPS WHICHCOMPRISES CLOSING SAID REGENERATOR AGAINST ESCAPE OF GAS, COMPRESSINGHEATED AIR INTO SAID REGENERATOR UNTIL THE PRESSURE EQUALS SUBSTANTIALLYTHE NORMAL OPERATING PRESSURE AT THE REGENERATOR AIR INLET WHEREBY THEPRESSURE AT THE REGENERATOR FLUE GAS OUTLET IS SUBSTANTIALLY ABOVE THENORMAL OPERATING PRESSURE, TRANSFERRING POWDERED CATALYST TO SAIDREGENERATOR IN SUSPENSION IN SAID AIR, AND DECREASING THE BACK PRESSUREON THE REGENERATOR FLUE GAS OUTLET AS THE CATALYST IS INTRODUCED THEREBYMAINTAINING A SUBSTANTIALLY CONSTANT PRESSURE DROP IN THE AIR PASSEDTHROUGH SAID REGENERATOR.